How Well Does Patient-reported Functional Disability Correspond to Measures of Physical Fitness in Adolescents with Fibromyalgia?

Abstract

The Functional Disability Inventory (FDI) is one of the most commonly used patient-reported outcome measures in clinical trials of pediatric pain. In addition to self-reported impairment, studies using activity monitoring devices show that adolescents with widespread musculoskeletal pain often exhibit sedentary behavior. Little is known about how impairment in daily physical activities is reflected in objectively-assessed physical fitness among adolescents with widespread pain. During a baseline assessment for a clinical trial, adolescents with juvenile fibromyalgia (N=226, Mage=15.26, 85% female) completed questionnaires (FDI, PROMIS-Fatigue, pain intensity VAS) and two validated assessments of physical functioning (The Harvard Step Test [HST], a test of aerobic fitness; and the Six-Minute Walk Test [6MWT], the distance a participant is able to walk in 6 minutes). We hypothesized that measures of physical fitness would predict variance in self-reported physical functioning (FDI scores) after controlling for pain and fatigue. We computed bivariate correlations followed by a hierarchical linear regression analysis with pain intensity and fatigue entered in the first step, fitness assessments in the second step, and FDI scores as the dependent variable. FDI scores were strongly associated with fatigue and pain levels (r=.55, p<.001 and r=.47, p<.001, respectively), but only modestly associated with HST (r=-.29, p<.001) and 6MWT scores (r=-.16, p=.02). In the regression model, pain and fatigue together explained 59% of the variance in FDI scores, while fitness measures explained only 2% unique variance (∆r2=.02, p=.07). HST significantly predicted FDI scores (β=-.16, t=-2.21, p=.03), but 6MWT distance did not. After controlling for pain intensity and fatigue, only the HST showed a significant relationship to self-reported functional disability. Overall, fitness measures contributed negligibly to FDI scores. Results support the need to examine cognitive and emotional factors (e.g., fear of movement) which may contribute to physical impairment among youth with chronic pain. Grant support from NIAMS/NIH R01 AR070474 (PI: Kashikar-Zuck).